This invention relates to an improved continuous rotary drum filter means. More specifically, the subject invention is related to a continuous rotary drum filter for the separation of petroleum liquids, such as lube oils, from solids, such as paraffins.
The separation of mixtures of compounds can be accomplished by various unit operations including distillation, solvent extraction and fractional crystallization. Fractional crystallization is particularly well suited to the purification of many mixtures economically in a one-stage operation. Frequently, the crystallization operation is combined with another unit operation to produce an improved separation process. In petroleum processing operations fractional crystallization is often utilized to separate waxy compounds from the lube oil fraction. Frequently, a solvent is added to the mixture to provide more favorable conditions for crystal growth and to dilute the resultant chilled slurry to thereby permit continuous oil-wax separation.
One type of filter particularly well suited to fractional crystallization, in general, and wax-lube oil separation, in particular, is the rotary drum filter. In this type filter, a filter drum continuously rotates within a filter vat containing the mixture to be separated. In one phase of the rotation, vacuum is applied to the filter drum causing a major portion of liquid to be drawn through the filter cloth and fluid conduits, while the solids and a minor amount of liquid are trapped by the filter cloth and build up to form a cake. During another phase of the rotation, high velocity gas is passed outwardly through the fluid conduits to purge the liquid and break the filter cake away from the cloth. During this purge cycle, liquid adhering to the walls of the conduits may be directed against sections of the filter cloth by the high velocity gas, thereby causing localized erosion of the filter cloth particularly near the ends of the filter drum. Since rotary filters frequently are thirty feet or more in length, filter cloth replacement is relatively expensive and necessitates complete shutdown of the filter.
One method for minimizing the erosive effects of the liquid on the filter cloth has been to have an elongated purge cycle in which the lead fluid conduit in the direction of flow is evacuated while the lag conduit, or conduit immediately following the lead conduit, is purged with high velocity gas. Liquid purged out of the lag pipe is directed into the lead pipe before it can strike the filter cloth. At a predetermined point in the rotation, the vacuum is discontinued and the purge gas and any remaining liquid is directed outwardly. Extension of the time period during which the combination of purge and evacuation is accomplished would decrease the amount of liquid remaining in the lag fluid conduit when evacuation of the lead fluid is discontinued. However, this would require a decrease in the rotational speed of the filter drum and/or a decrease in the other filter drum cycle times. Either of these methods thus would decrease the filtration capacity of the unit.
Yet another method of decreasing the erosive effects of the liquid would be to use a thicker or more erosion-resistant filter cloth. Use of a thicker filter cloth would increase the filter cloth cost and also increase the tendency of the filter cloth to bow while also decreasing the filter capacity. In addition, the thicker the filter cloth, the more difficult it is to remove the wax fines from the cloth. Use of a more erosion-resistant filter cloth may not be advantageous, since a more erosion-resistant cloth may not have desirable filter cake discharge characteristics.
Accordingly, it is desirable to produce a modified rotary filter in which erosion of the filter cloth by entrained liquid is reduced or eliminated without decreasing the filter throughput or increasing the cost of replacement filter cloths.
It is also desirable to have a rotary filter design which minimizes erosion and which is relatively inexpensive and reliable.
It is also desirable to have a rotary filter design which is easily adaptable to rotary filters currently in use for decreasing the rate of filter cloth erosion.